Webs of material are often used in various industries and in a variety of ways. These materials can include paper, plastic, and other materials formed or processed in sheets. As a particular example, long webs of paper or other material can be manufactured and collected in reels. These webs of material are often manufactured or processed at a high rate of speed, such as speeds of up to one hundred kilometers per hour or more.
It is often necessary or desirable to measure one or more properties of a web of material as the web is being manufactured or processed. For example, it is often desirable to measure the properties of a paper web (such as its basis weight, moisture, color, or caliper/thickness) to verify whether the web is within certain specifications. Adjustments can then be made to the web-making process to ensure the web properties are within the desired range(s).
Many optical- and image-based measurement systems require that a web be illuminated in a specific manner. For example, certain measurements may require that a web be illuminated with light having specific spectral characteristics or at a specific angle. Changes in the spectral characteristics or angle of illumination can prevent proper measurements of the web's properties.